Electromagnetic wave transmission cable including a hollow dielectric tube surrounded by a foamed resin member having different expansion ratios at different regions therein

ABSTRACT

An electromagnetic wave transmission cable for transmitting an electromagnetic wave comprises a hollow waveguide tube and a foamed resin member. The hollowing waveguide tube includes a hollow dielectric layer formed in a tubular shape. The foamed resin member is provided over a predetermined length in a longitudinal direction of the hollow waveguide tube and covers a surface of the dielectric layer to surround an outer periphery of the hollow waveguide tube.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage entry of PCT Application No:PCT/JP2017/037700 filed Oct. 18, 2017, which claims priority to JapanesePatent Application No. 2016-232105, filed Nov. 30, 2016, the contents ofwhich are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an electromagnetic wave transmissioncable.

BACKGROUND ART

A hollow waveguide tube made of a dielectric such as a resin has a lightweight, high flexibility, a low transmission loss, and high transmissionefficiency compared to metal cables such as a coaxial cable and metalwaveguide tubes, and is thus effective as a cable for mainlytransmitting millimeter waves (30 to 300 GHz) to THz-band (0.1 to 100THz) electromagnetic waves.

While a hollow waveguide tube made of a single-layer dielectric tube hasa confinement effect with respect to the surrounding air, theconfinement effect can be hampered and electromagnetic waves can leakand scatter off the dielectric tube if a metal or another dielectric,such as a human body in particular, comes into contact with the outerwall of the waveguide tube.

A transmission path that confines and transmits electromagnetic waves bystacking two types of dielectrics in layers to construct a Bragg mirroron the outer periphery of a waveguide tube has thus been conceived (forexample, Patent Literature 1).

CITATION LIST Patent Literature

Patent Literature 1: United States Patent Application Publication No.2009/0097809 published on Sep. 4, 2019.

SUMMARY OF THE INVENTION Technical Problem

According to the foregoing conventional technique, different materialsneed to be stacked in layers to constitute the Bragg mirror. There havethus been problems of increased number of processes and highmanufacturing cost. In addition, the thicknesses of the respectivelayers need to be designed on the basis of the wavelength oftransmission, and there has been a problem of wavelength dependence.

Among examples of problems to be solved by the present invention is thatthe electromagnetic wave confinement effect is hampered when anotherobject is in contact with the hollow waveguide tube.

Solution to the Problem

The invention is an electromagnetic wave transmission cable fortransmitting an electromagnetic wave, comprising: a hollow waveguidetube including a hollow dielectric layer formed in a tubular shape; anda foamed resin member that is provided over a predetermined length in alongitudinal direction of the hollow waveguide tube and covers a surfaceof the dielectric layer to surround an outer periphery of the hollowwaveguide tube.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically showing an electromagnetic wavetransmission cable according to the present invention.

FIG. 2 is a longitudinal cross-sectional view of the electromagneticwave transmission cable according to the present invention.

FIGS. 3A, 3B, and 3C are a longitudinal cross-sectional views showingmodifications of the electromagnetic wave transmission cable accordingto the present invention.

FIG. 4 is a diagram schematically showing a modification of theelectromagnetic wave transmission cable according to the presentinvention.

FIG. 5 is a longitudinal cross-sectional view showing a modification ofthe electromagnetic wave transmission cable according to the presentinvention.

FIGS. 6A and 6B are longitudinal cross-sectional views showing amodification of the electromagnetic wave transmission cable according tothe present invention.

FIG. 7 is a diagram schematically showing an electromagnetic wavetransmission cable according to a second embodiment.

FIGS. 8A, 8B, and 8C are longitudinal cross-sectional views of theelectromagnetic wave transmission cable according to the secondembodiment.

FIG. 9 is a chart showing an example of a relationship between thepresence or absence of an outer covering and a transmission loss.

FIG. 10 is a chart showing a relationship between the presence orabsence of the outer covering and an effective refractive index.

DETAIL DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below withreference to the drawings. In the following description and theaccompanying drawings, substantially the same or equivalent parts aredesignated by the same reference numerals.

First Embodiment

FIG. 1 is a diagram schematically showing a configuration of anelectromagnetic wave transmission cable 100 according to the presentembodiment. The electromagnetic wave transmission cable 100 includes ahollow waveguide tube 10 including a hollow dielectric layer in which awaveguide of tubular shape is formed, and a foamed resin member 20covering the outside surface of the hollow waveguide tube 10. Thedielectric layer and the hollow portion of the hollow waveguide tube 10form a waveguide for transmitting electromagnetic waves EW.

FIG. 2 is a longitudinal cross-sectional view of the electromagneticwave transmission cable 100. The hollow waveguide tube 10 includes adielectric layer 11 and a hollow region 12.

The dielectric layer 11 is formed in a tubular shape to surround thehollow region 12 with a center axis at the center thereof. In otherwords, the dielectric layer 11 has a C rotationally symmetrical shapeabout the center axis CA. For example, FIG. 1 shows a case where thecross section of the dielectric layer 11 in a direction perpendicular tothe center axis CA has a circular outer rim, whereas the outer rim mayhave an oblong circular shape, an elliptical shape, a rectangular shape,etc. The dielectric layer 11 is made of a fluorocarbon resin PTFE(polytetrafluoroethylene), for example. The dielectric layer 11 thus hasa refractive index of approximately 1.5.

The hollow region 12 is formed in a rotationally symmetrical shape aboutthe center axis CA along the inner diameter of the hollow waveguide tube10 (i.e., the inner diameter of the dielectric tube made of thedielectric layer 11). Note that the dielectric waveguide does notnecessarily need to have a rotationally symmetrical shape as long asdesired performance is obtained. For example, while FIG. 1 shows a casewhere the cross section of the hollow region 12 (i.e., the cross sectionof the waveguide) in a direction perpendicular to the center axis CA iscircular, the cross section may have an oblong circular shape, anelliptical shape, a rectangular shape, etc.

There is a close relationship between the wavelength of theelectromagnetic waves EW flowing through the hollow waveguide tube 10and an optimum tube shape. For example, to enhance adherence to the HE11mode of low transmission loss, the inner diameter of the hollowwaveguide tube 10 is desirably set to be smaller than the wavelength. Onthe other hand, the inner diameter of the hollow waveguide tube 10 isdesirably set to be greater than the half-wavelength since theelectromagnetic wave confinement effect weakens as the hollow region 12decreases. The inner diameter of the hollow waveguide tube 10 accordingto the present embodiment is therefore set to be equal to or greaterthan the half-wavelength and not greater than the wavelength.

The outer diameter of the hollow waveguide tube 10 is desirably greaterthan a wavelength equivalent (wavelength×refractive index). On the otherhand, to make the transmission in the HE11 mode dominant, the outerdiameter of the hollow waveguide tube is desirably set to be not solarge. Specifically, the outer diameter is desirably set to be less thanor equal to twice an equivalent wavelength. The outer diameter of thehollow waveguide tube 10 according to the present embodiment istherefore set to be equal to or greater than the equivalent wavelengthand not greater than twice equivalent wavelength equivalent.

To enhance the adherence to the HE11 mode of low transmission loss, thethickness of the dielectric layer 11 of the hollow waveguide tube 10 isdesirably set to be smaller than the wavelength. However, since adielectric layer 11 that is too thin fails to provide strength neededfor supporting the waveguide, the thickness of the dielectric layer 11is desirably set to be greater than 1/10 of the wavelength. Thethickness of the dielectric layer 11 according to the present embodimentis therefore set to be equivalent to or greater than 1/10 of thewavelength and not greater than one wavelength.

The foamed resin member 20 extends in the longitudinal direction of thehollow waveguide tube 10 (i.e., waveguide direction) and covers theoutside surface of the dielectric layer 11 (i.e., surface opposite fromthe hollow region 12) to surround the outer periphery of the hollowwaveguide tube 10 with the center axis CA of the hollow waveguide tube10 at the center. The foamed resin member 20 in its simplest form has arotationally symmetrical cross-sectional shape about the center axis CAof the hollow waveguide tube 10. However, the foamed resin member 20 mayhave any shape as long as desired performance is obtained. For example,while FIG. 1 shows a case where the cross section of the foamed resinmember 20, in the direction perpendicular to the center axis CA, has acircular outer rim, the outer rim may be an oblong circle, an ellipse, arectangular, etc. The outer rim of the cross section of the foamed resinmember 20 may have the same shape as or a different shape from that ofthe outer rim of the cross section of the dielectric layer 11.

For example, the foamed resin member 20 is made of foamed polystyrene.Foamed polystyrene has a fine intricate structure of polystyrene havinga refractive index of 1.6 compared to air having a refractive index of1, and includes a lot of fine reflection interfaces between polystyrenethat is the dielectric and air. For example, low-expansion-ratio foamedpolystyrene used as a packaging material was measured in a bulk stateand found to have an average refractive index (refractive index based onthe assumption that the bulk material was a uniform medium of a singlesubstance) of approximately 1.1 at 0.1 to 0.5 THz. This resultdemonstrates that a large amount of air is mixed in foamed polystyrene.

As described above, the foamed resin member 20 contains a large amountof air, and the ratio of polystyrene which is in contact with thesurface of the hollow waveguide tube 10 is extremely low. This cansignificantly reduce the leakage of electromagnetic waves even insituations where a metal, a human body, or the like comes into contactwith the outside surface of the foamed resin member 20 (i.e., surfaceopposite from the surface which is in contact with the hollow waveguidetube 10).

A thickness of the foamed resin member 20 which is too small lowers theelectromagnetic wave confinement effect. For example, in an experimentperformed by using a PTFE hollow waveguide tube having a waveguidefrequency of 300 GHz, an outer diameter of 0.9 mm, and an inner diameterof 0.5 mm, a sufficient effect was not obtained if the thickness of thefoamed resin member 20 was less than 1 mm. The thickness of the foamedresin member 20 (thickness of the covering portion) is thereforedesirably set to be greater than or equal to a thickness equivalent tothe wavelength of the electromagnetic waves to be transmitted(wavelength×average refractive index). Since a thickness that is toogreat results in poor handleability, the thickness of the foamed resinmember 20 is desirably set to 50 mm or less, preferably 10 mm or less.

As described above, in the electromagnetic wave transmission cable 100according to the present embodiment, the surface of the hollow waveguidetube 10 is covered with the foamed resin member 20. The foamed resinmember 20 contains a large amount of air and has an average refractiveindex lower than the refractive index of the dielectric layer 11 of thehollow waveguide tube 10.

With such a configuration, the electromagnetic wave confinement effectof the hollow waveguide tube 10 can be maintained even in situationswhere a metal, human body, or other object comes into contact therewith.

The embodiment of the present invention is not limited to the foregoingembodiment. For example, in the foregoing embodiment, the foamed resinmember 20 is described to be made of foamed polystyrene. However, thematerial of the foamed resin member 20 is not limited thereto, and thefoamed resin member 20 may be made of foamed polyurethane, foamedpolyolefin (foamed polyethylene, foamed polypropylene), foamedpolytetrafluoroethylene (PTFE), or the like.

If the outer diameter of the hollow waveguide tube 10 is smaller thanthe equivalent wavelength (i.e. wavelength×refractive index), thereoccurs a wave component propagating over the outer periphery of thewaveguide tube. This can cause an adverse effect if the covering foamedresin has an attenuation factor higher than that of air. If the materialof the foamed resin member 20 is polystyrene (PS), polyethylene (PE), orfluorocarbon resin (PTFE), the foamed material has an averageattenuation factor of 0.1 cm-1 or less, and such adverse effects areless likely to occur.

In the foregoing embodiment, the foamed resin member 20 is described tobe made of foamed polystyrene and has an average refractive index ofapproximately 1.1 at 0.1 to 0.5 THz, for example. However, the averagerefractive index of the foamed resin member 20 is not limited thereto.Since the electromagnetic wave confinement effect results from a lowrefractive index, the foamed resin member 20 desirably has a highexpansion ratio. However, an expansion ratio that is too high increasessoftness and results in poor handleability. The foamed resin member 20is therefore desirably foamed to an extent such that the averagerefractive index in the transmission frequency band in a bulk statefalls below 1.4.

Unlike the foregoing embodiment, the dielectric layer 11 of the hollowwaveguide tube 10 and the foamed resin member 20 may be made of the sametype of material by using a foam of foamed fluorocarbon resin(polytetrafluoroethylene) (PTFE) as the material of the foamed resinmember 20. In other words, the hollow waveguide tube 10 and the foamedresin member 20 can be constituted by changing the expansion ratio ofthe same material.

A three-dimensional structure may be formed on the surface of the foamedresin member 20. For example, the electromagnetic wave transmissioncable 100 can be made flexible by forming a protruding structure on theoutside surface opposite from the inside surface that is in contact withthe dielectric layer 11 as shown in FIG. 3A. The electromagnetic waveconfinement effect can be enhanced by forming a protruding structure onthe inside surface that is in contact with the dielectric layer as shownin FIG. 3B. The three-dimensional structure formed on the surface(s) ofthe foamed resin member 20 may have a notch shape or otherpit-and-projection shape.

To protect the hollow waveguide tube 10 and the foamed resin member 20from collapsing, as shown in FIG. 3C, an outer coating 30 covering theoutside surface of the foamed resin member 20 may be provided along thelongitudinal direction of the hollow waveguide tube 10 and the foamedresin member 20.

As shown in FIG. 4, instead of covering the entire hollow waveguide tube10 with the foamed resin member 20, only parts that can come intocontact with other members, such as connector units for connectingwaveguide tubes to each other and supports for holding the waveguidetube(s) at a predetermined height, may be configured to be covered withthe foamed resin member 20. In other words, the foamed resin member 20may be formed over a predetermined length (distance) in the longitudinaldirection of the hollow waveguide tube, and may be provided at aplurality of positions.

As shown in FIG. 5, the expansion ratio of the foamed resin member 20may be changed stepwise between regions closer to a contact surface thatis in contact with the dielectric layer 11 (i.e., inside) and regionsfarther from the contact surface (i.e., outside). For example, in theinside regions closer to the contact surface with the dielectric layer11, the expansion ratio can be increased (i.e. large) to reduce therefractive index and enhance the electromagnetic wave confinementeffect. In the outside regions farther from the contact surface with thedielectric layer 11, the expansion ratio can be reduced (i.e. small) toincrease cable rigidity. This can be implemented, for example, bydividing the regions closer to and farther from the contact surface ofthe foamed resin member 20 with the dielectric waveguide 10 into aplurality of areas, and reducing the expansion ratios of the respectiveareas stepwise from the areas closer to the contact surface to thefarther areas.

As shown in FIGS. 6A and 6B, a foamed resin member 20 can be used as aconnector 40 (FIG. 6A) in a connector unit for connecting waveguidetubes to each other. For example, as shown in FIG. 6A, by utilizing theproperty of the foamed resin member 20 being deformable, foamed resinmembers 20 are provided on the inner sides of a pair of holding members41A and 41B to form a tapered gap. As shown in FIG. 6B, the hollowwaveguide tube 10 can thus be inserted into the connector 40 so that thehollow waveguide tube is supported. This enables easy and reliable cablepositioning while preventing the electromagnetic wave confinement effectfrom being hampered by contact with the holding members 41A and 41B.

In the electromagnetic wave transmission cable 100 according to thepresent embodiment, the dielectric layer 11 may be made of e-PTFE(expanded polytetrafluoroethylene) which is PTFE formed by stretchprocessing. Note that e-PTFE can be obtained, for example, by stretchinga PTFE material at least in one direction to provide continuous porosity(i.e. structure including a large number of continuous pores) and thensintering-fixing (i.e. fixing by sintering) the resultant material athigh temperature. The stretched porous resin (e-PTFE) used in thepresent embodiment has characteristic fine nodes and fine fiberstructures in the stretching direction, and can thus function as amedium having a low average refractive index without increasing theelectromagnetic wave transmission loss.

The porosity of the stretched porous resin (the proportion of porousportions in the resin) can be selected from among 30% to 90% dependingon the intended use. To cover the outside with the foamed resin member20 as in the present embodiment, there needs to be a refractive indexdifference within the foamed resin member 20. To suppress theelectromagnetic wave transmission loss, a refractive index difference ofat least 0.01 or so is need. The desirable porosity derived therefrom is70% or less. The optimum range of the porosity of the stretched porousresin of the dielectric layer 11 according to the present embodiment istherefore 30% to 70%.

Second Embodiment

FIG. 7 is a diagram schematically showing a configuration of anelectromagnetic wave transmission cable 200 according to the presentembodiment. The electromagnetic wave transmission cable 200 includes ahollow waveguide tube 10 including a hollow dielectric layer 11 in whicha waveguide of tubular shape is formed, a foamed resin member 20covering the outside surface of the hollow waveguide tube 10, and ametal film 50 covering the outside surface of the foamed resin member20.

FIG. 8A is a longitudinal cross-sectional view of the electromagneticwave transmission cable 200. The hollow waveguide tube 10 includes adielectric layer 11 and a hollow region 12.

The dielectric layer 11 is made of a resin material having a lowrefractive index or a low complex refractive index, such as PTFE(polytetrafluoroethylene), e-PTFE (expanded polytetrafluoroethylene)formed by stretching PTFE to provide continuous porosity and fixing thesame by sintering, and PE (polyethylene). To reduce a transmission loss,the tube (i.e. dielectric layer 11) desirably has a small thickness.

The foamed resin member 20 is arranged to surround the outer peripheryof the hollow waveguide tube 10. The foamed resin member 20 needs tohave a refractive index lower than that of the dielectric constitutingthe dielectric layer 11 of the hollow waveguide tube 10. The closer to 1the refractive index is, the better.

The foamed resin member 20 is formed on the outer peripheral surface ofthe hollow waveguide tube 10, for example, by a method of inserting thehollow waveguide tube 10 into the inside of a foamed resin of hollowshape, a method of winding foamed resin around the hollow waveguide tube10, or other methods.

Like the hollow waveguide tube 10 and the foamed resin member 20, themetal film 50 has a rotationally symmetrical shape about the center axisCA. The metal film 50 extends in the longitudinal direction of thehollow waveguide tube 10 and the foamed resin member 20, and furthercovers the outside surface of the foamed resin member 20 covering thehollow waveguide tube 10 to surround the outer periphery of the foamedresin member 20 with the center axis CA at the center. Note that themetal film 50 may have any shape as long as desired performance isobtained. While FIG. 7 shows a case where the cross section of the metalfilm 50, in a direction perpendicular to the center axis CA, has acircular outer rim, the outer rim may be an oblong circle, an ellipse, arectangular, etc. The outer rim of the cross section of the metal film50 may have the same shape as or a different shape from that of thecross section of the hollow waveguide tube 10 or the foamed resin member20. In other words, the metal film 50 may have any shape covering thefoamed resin member 20.

The metal film 50 is made of a metal having a relatively highconductivity, such as gold, silver, and copper. The metal film 50 mayhave a thickness of about 1 μm or more. The metal film 50 is formed, forexample, by a method of directly forming a metal film on the surface ofthe foamed resin member 20, a method of forming a metal film on thesurface of a dielectric sheet to fabricate a metal-coated sheet inadvance and then winding the metal-coated sheet on the foamed resinmember 20 with the metal surface toward the foamed resin member 20, orother methods.

The outer peripheral portion of the metal film 50 is actually coveredwith a protective film 60 made of a dielectric or the like, as shown inFIG. 8B. The protective film has only a protective function and does notcontribute to electromagnetic wave transmission. For example, if themetal film 50 is formed by the method of winding a metal-coated sheet onthe foamed resin member 20, the dielectric sheet can be left unremovedand used as a protective film.

As described above, in the electromagnetic wave transmission cable 200according to the present embodiment, the metal film 50 is formed tofurther cover the outside surface of the foamed resin member 20 coveringthe hollow waveguide tube 10. The effect of the metal film 50 will bedescribed with reference to FIGS. 9 and 10.

FIG. 9 is a graph showing a relationship between the thickness in nm ofthe dielectric layer 11 (dielectric tube) made of e-PTFE and thetransmission loss, depending on the presence or absence of the outercovering.

If the hollow waveguide tube is formed alone without any outer covering,as shown by a broken line in FIG. 9, the transfer or transmission lossdecreases as the thickness of the dielectric layer decreases (dielectrictube becomes thinner).

If the outside surface of the dielectric layer of the hollow waveguidetube is covered with a foamed polystyrene resin having an expansionratio of approximately 30 times, as shown by a dot-dashed line in FIG.9, the transmission loss is lower than without an outer covering, andthe transmission loss decreases with the decreasing thickness of thedielectric layer within a range where the dielectric layer (dielectrictube) has a certain thickness or more (in FIG. 10, approximately 0.2 mmto 0.7 mm). However, if the thickness of the dielectric layer fallsbelow a certain value (0.2 mm), the transmission loss degrades to anontransmissible degree (in FIG. 9, shown as immeasurable).

In contrast, if the outside surface of the foamed resin covering thehollow waveguide tube is further covered with a metal film, as shown bya solid line in FIG. 9, the transmission loss decreases in proportion tothe thickness of the dielectric layer. The transmission loss does notdegrade even in the range where the thickness of the dielectric layer isless than the certain value (0.2 mm).

FIG. 10 is a graph showing a relationship between the thickness in nm ofthe dielectric layer and an effective refractive index, depending on thepresence or absence of the outer covering. The effective refractiveindex is a parameter that represents an average refractive index of themedium contributing to the electromagnetic wave transmission andcharacterizes the transmission state of the electromagnetic waves.

If the outside surface of the dielectric layer of the hollow waveguidetube is covered with a foamed resin, as shown by a dot-dashed line inFIG. 10, the effective refractive index asymptotically approaches therefractive index of the foamed resin (1.016) (in the case of foamedpolystyrene having an expansion ratio of approximately 30 times) as thethickness of the dielectric layer decreases (dielectric tube becomesthinner). This indicates that the propagation medium of theelectromagnetic waves shifts from the dielectric to the foamed resin asthe thickness of the dielectric layer decreases. The transmission withthe foamed resin as the propagation medium is unstable in terms of theconfinement of the electromagnetic waves, and eventually becomesincapable of transmission.

In contrast, if the outside surface of the foamed resin is covered withthe metal film, as shown by a solid line in FIG. 10, the effectiverefractive index does not asymptotically approach the refractive indexof the foamed resin, but the effective refractive index decreases withthe decreasing thickness of the dielectric layer. This indicates thatthe propagation medium of the electromagnetic waves does not shift tothe foamed resin and the electromagnetic waves are stably transmittedwith the dielectric layer (dielectric tube) as the waveguide.

As described above, in the electromagnetic wave transmission cable 200according to the present embodiment, the outer covering of the hollowwaveguide tube 10 has a double structure including the foamed resinmember 20 and the metal film 50. This can suppress a drop in thetransmission loss occurring if the covering is the foamed resin alone.More specifically, if the dielectric layer of the hollow waveguide tubeis covered with only the foamed resin, electromagnetic wave transmissioncan no longer be transmitted once the transmission loss falls below alower limit value determined by the physical property values of thedielectric waveguide and the foamed resin (for example, the effectiverefractive index falls below the refractive index of the foamed resin).However, if the foamed resin (foamed resin member 20) is further coveredwith the metal film as in the present embodiment, transmission lossesbelow the lower limit value (lower limit value of the transmission lossin the case of covering with only the foamed resin) can be achieved.

In the electromagnetic wave transmission cable 200 according to thepresent embodiment, the metal film 50 functions as a shield againstexternal electromagnetic waves (i.e. noise). Since the hollow waveguidetube 10 is shielded from outside effects, electromagnetic waves can bestably transmitted.

The thickness of the entire covering portion can be reduced forminiaturization, compared to when the hollow waveguide tube is coveredwith only a foamed resin.

The metal film 50 does not necessarily need to cover the entire outsidesurface of the foamed resin member 20. For example, the metal film 50may be patterned to partly cover the outside surface of the foamed resinmember 20 at intervals less than or equal to the wavelength of theelectromagnetic waves EW. The patterning of the metal film may beimplemented, for example, by a method of winding a metal thin wire or amethod of putting a net such as a braided shield. The patterning of themetal film may be implemented by forming a wire grid or a metamaterialstructure by using a mask during film formation.

The needed thickness of the foamed resin member 20 can be reduced byadjusting the structures and materials of the dielectric waveguide 10and the metal film 50.

While the metal film 50 is described to be made of a metal such as gold,silver, and copper, the metal film 50 may be made of aluminum or analloy. The metal constituting the metal film 50 desirably has a highconductivity, whereas any metal having some conductivity can be usedsince even a metal having a low conductivity has the effect of improvingthe transmission characteristic.

Instead of the metal film 50, a dielectric film 51 may be used as theouter coating that covers the foamed resin member 20, as shown in FIG.8C. For example, a dielectric film having a refractive index of about1.4 or more can provide a sufficient refractive index difference at theinterface.

The electromagnetic wave transmission cable according to the presentembodiment can be used, for example, as a cable for in-vehicle highcapacity high speed information communication to replace a vehicleinformation harness of an automobile and the like, or as a cableintended for a data center, moving image transmission, and the likewhere high capacity communication is needed.

The invention claimed is:
 1. An electromagnetic wave transmission cablefor transmitting an electromagnetic wave, comprising: a hollow waveguidetube including a hollow dielectric layer formed in a tubular shape; anda foamed resin member that is provided over a predetermined length in alongitudinal direction of the hollow waveguide tube and covers a surfaceof the dielectric layer to surround an outer periphery of the hollowwaveguide tube, wherein the foamed resin member has different expansionratios between a region of the foamed resin member close to a contactsurface of the foamed resin member that is in contact with thedielectric layer and a region of the foamed resin member far from thecontact surface.
 2. The electromagnetic wave transmission cableaccording to claim 1, wherein the foamed resin member has a rotationallysymmetrical sectional shape about a center axis of the hollow waveguidetube.
 3. The electromagnetic wave transmission cable according to claim1, wherein the dielectric layer is made of a dielectric containing amaterial having stretch porosity.
 4. The electromagnetic wavetransmission cable according to claim 1, wherein the different expansionratios within the foamed resin member have a high expansion ratio in theregion close to the contact surface and a low expansion ratio in theregion far from the contact surface.
 5. The electromagnetic wavetransmission cable according to claim 1, wherein the contact surface ofthe foamed resin member covers the surface of the dielectric layer at aplurality of positions in the longitudinal direction of the hollowwaveguide tube.
 6. The electromagnetic wave transmission cable accordingto claim 1, wherein the contact surface of the foamed resin memberincludes a protruding structure on an inner surface thereof that is incontact with the dielectric layer.
 7. The electromagnetic wavetransmission cable according to claim 1, wherein the foamed resin memberincludes a protruding structure on an outside surface thereof oppositefrom the contact surface.
 8. The electromagnetic wave transmission cableaccording to claim 1, further comprising an outer coating that covers anoutside surface of the foamed resin member along the longitudinaldirection of the hollow waveguide tube and the foamed resin member. 9.The electromagnetic wave transmission cable according to claim 8,wherein the outer coating is a dielectric.
 10. The electromagnetic wavetransmission cable according to claim 8, wherein the outer coating is ametal film.
 11. The electromagnetic wave transmission cable according toclaim 10, further comprising a protective film that covers an outsidesurface of the outer coating along the longitudinal direction of thehollow waveguide tube.
 12. The electromagnetic wave transmission cableaccording to claim 11, wherein the protective film is made of a materialcontaining a dielectric material.
 13. The electromagnetic wavetransmission cable according to claim 1, wherein the foamed resin memberis made of any one of foamed polystyrene, foamed polyurethane, foamedpolyolefin, and foamed polytetrafluoroethylene.
 14. The electromagneticwave transmission cable according to claim 1, wherein the foamed resinmember is made of the same material as that of the dielectric layer. 15.The electromagnetic wave transmission cable according to claim 1,wherein the electromagnetic wave has a frequency of 30 to 100000 GHz.16. The electromagnetic wave transmission cable according to claim 1,wherein the foamed resin member has an average refractive index of 1 ormore and not more than 1.4 in a transmission frequency band of theelectromagnetic wave.
 17. The electromagnetic wave transmission cableaccording to claim 1, wherein the foamed resin member covering thedielectric layer has a thickness greater than or equal to an equivalentwavelength expressed by a wavelength of the electromagnetic wavemultiplied by an average refractive index of the foamed resin member.18. The electromagnetic wave transmission cable according to claim 1,wherein the foamed resin member has an average attenuation factor of 0.1cm⁻¹ or less with respect to the electromagnetic wave.
 19. Theelectromagnetic wave transmission cable according to claim 1, whereinthe hollow waveguide tube has an outer diameter greater than or equal toan equivalent wavelength expressed by a wavelength of theelectromagnetic wave multiplied by a refractive index of the dielectriclayer.